The aerodynamic resistance can be calculated with and without stability correction (see switch Aerodyn.Resistance). Without stability correction the aerodynamic resistance is calculated as:

where the wind speed, *u*, is given at the reference height, *z _{ref}*,

If the aerodynamic resistance is calculated as a function
of the Richardson’s number, eq. (3.15) is
multiplied by the Richardson’s stability function as described in eq.
(4.14)-(4-17). The stability correction can also be accounted for by calculating
the aerodynamic resistance by the Monin-Obukhov stability function (eq. 4.18)
instead of eq.(3.15). In both
cases the roughness length used in the calculation of *r _{a}* is
the roughness length calculated for each plant (i.e. eq. (3.17)) and the
parameter

If more than one canopy
exist (see Description of
Plant) additional
contributions to the aerodynamic resistance will be estimated because of
eventual shadowing of other canopies. The aerodynamic resistance for a specific
canopy (*i*) is then calculated as:

where
*p _{ral}* is a parameter and

When simulating an explicit single big leaf plant the
roughness length, *z _{o}*, can either be given
in a PG-file, read from a parameter table or estimated by functions following
data presented by Shaw and Pereira (1982) (see Roughness). For multiple plants the roughness length
is either calculated by the Shaw and Pereira function or is estimated by linear
functions (see Roughnessfunc).

The Shaw and Pereira function calculate the roughness length as:

where
*z _{0max}* and

and *A** _{PAI}* is the plant area
index, which is defined as the sum of leaf area index,

If snow is included in the simulation, the function for estimating roughness has to be adjusted in the following way:

where
*Δz _{snow}* is the snow depth.

If
roughness is determined by linear functions, *f _{1}* and

Also the displacement height, *d*, can be given in a
PG file, read from a parameter table, or estimated by a function derived from
suggestions presented by Shaw and Pereira (1982) (see switch Displacement). For
multiple plants displacement is either calculated by the Shaw and Pereira
function or is estimated by linear functions (eq.(3.3)) (see Roughnessfunc).

The Shaw and Pereira function calculates the displacement height as:

See viewing function Displacement height, Shaw and Pereira.

If snow is included in the simulation, the function for estimating displacement height has to be adjusted in the following way:

where
*Δz _{snow}* is the snow depth.

If the displacement height is determined by linear functions, eq.(3.20) is modified into:

The
linear function, *f _{3}*, is calculated by eq.(3.3) and
values found in the parameter table Displacement
coefficients – multiple canopies. Eq.(3.21)is modified
analogously. See viewing function Displacement height, linear function.